Lamps of the type to which the invention relates are known for instance from German Patent Disclosure DE-OS 22 31 520. Their cold fill pressure is typically approximately 5 to 15 atm of an inert gas, predominantly noble gases (argon, krypton, xenon), which optionally have a slight proportion (5 to 10%) of nitrogen. Traces of one or more halogen compounds are also added, whose partial pressure amounts to only a few millibars.
In such lamps, predominantly for general lighting purposes, the attainment of a relatively long service life (AB: 2000 hours, FO 200 hours and more) is a major goal. To achieve this, the assumption is generally made that the evaporation rate of the tungsten material of the luminous element must be damped by means of the highest possible fill pressure (in halogen lamps approximately 2.7 to 10.7 bar (approximately 2000 Torr), according to H. Lohmann, Electrotechnik Electrical Engineering!, 1986, pp. 33-36 and particularly page 35. At the same time, the halogen compound present as a fill component, with the aid of the convection occurring in the bulb, supports a cycle process for the tungsten particles evaporating from the luminous element (S. M. Correra, Int. J. Heat Mass Transfer 30, p. 663, 1987). However, the convection also causes a considerable heat loss, which is on the order of magnitude of 10% of the lamp power.
In general, it has until now been considered indispensible to maintain such operating conditions, in order to be able to achieve a high light yield (up to 25 lm/W) with a simultaneously high lamp life (at least 2000 hours).
For special reasons, it is true that special lamps with a low cold fill pressure have been individually developed (European Patent Disclosure DE-A 295 592), but on the precondition that nevertheless the convection that drives the halogen cycle takes place. A further example of this is French Patent Disclosure FR-A 2 436 495. It describes a photographic lamp pinched on both ends, in which the cold fill pressure is lowered to about 0.2 bar in order to reduce the risk of explosion. This is achieved at the usual cost of reducing the service life. Moreover, in U.S. Pat. No. 4,463,277, an incandescent halogen lamp with a cold fill pressure of about 0.8 to 0.93 bar is described. The low pressure is intended to enable the use of hard glass.
An essential role in the theoretical discussion of incandescent lamps is played by the concept of the Langmuir layer, which has been explained in detail, for instance in Techn. Wissenschaftl. Abhandlungen der OSRAM GmbH (TWAOG) Industrial Science Papers published by OSRAM GmbH!, Vol. 9, pp. 125-136, 1967, Springer-Verlag, Berlin. Here the existence of a horizontal stationary gas layer, in fact the Langmuir layer, adjacent to the luminous element, which is considered to be a cylinder, is assumed, and is observed to be homogeneous and of constant diameter. In it, heat removal takes place solely by heat dissipation, while outside this layer the heat losses are determined by free convection. In incandescent halogen lamps, the Langmuir layer thickness is on the order of magnitude of a few millimeters (see also the aforementioned German Patent Disclosure DE-OS 22 31 520). It is dependent on the fill pressure.
From European Patent Disclosure EP-A 0 241 911, an incandescent halogen lamp with a luminous element with a wire diameter on the order of magnitude of 100 .mu.m is known.
Special conditions pertain in soffit lamps, that is, elongated incandescent lamps pinched on both ends with an axially arranged luminous element. Here, if there is a deviation from the horizontal position (and particularly in a vertical burning position), severe problems arise in operation, having to do with the demixing that occurs between the fill gas and the halogen additive inside the bulb. In U.S. Pat. No. 3,435,272 and in an article in Illum. Engin., April 1971, pp. 196-204, the cooperation of diffusion and convection at a fill pressure of 0.5 to 15 bar is; said to be responsible for this thermally-caused demixing effect. This effect is suppressed by means of a glass tube introduced into the bulb and surrounding the luminous element. A similar concept is also presented in U.S. Pat. No. 4,703,220. Here the glass tube is intended to partially suppress convection.
The invention describes an entirely new way to furnish incandescent halogen lamps as defined by the preamble to claim 1, which makes possible a relatively high light yield and at the same time a long lamp life.
This novel technical teaching is based on the characteristics of the body of claim 1. Especially advantageous features are found in the dependent claims.
While the prevailing teaching was based on a linear relationship between the fill pressure and service life, entirely surprisingly, it has also been demonstrated that under certain peripheral conditions, namely in lamps with thin wires as the luminous element material, values for the light yield and service life can be attained that are even equivalent to those at high pressures. The wires preferably have a maximum diameter of 200 .mu.m, and especially preferably-less than 100 .mu.m. Excellent improvements can be attained in lamps with wire diameters of below 50 .mu.m. The cause for this performance is that with small wire diameters, instead of the evaporation mechanism, some other failure mechanism determines the service life of the wire, namely the migration of the tungsten along the wire. At very small wire diameters, the grain structure of the wire makes itself strongly felt, since only from one to two grains are present across the wire diameter. The migration of the tungsten makes itself felt at the grain boundaries in the form of constrictions between the grains. This process is known as "grain boundary corrosion". This failure mechanism is independent of the fill pressure; on the other hand, it is highly dependent on the surface temperature of coiled wire (typical temperature values are approximately 2300 to 3200K). Under the operating conditions assumed here, this failure mechanism is surprisingly sharply reduced.
A decisive consequence of the altered failure behavior is the correspondingly altered failure characteristic. Typically, the failure behavior of a number of lamps follows a modified Gaussian distribution, which is known as a Weibull distribution. It is characterized by the mean value (time period until failure of 63.2% of a set of lamps) and a certain deviation range (variance). Until now, for a mean value of typically 7000 hours, this variance was typically 5000 hours.
In other words, the variance was very large in comparison with the mean value. By comparison, in lamps according to the invention, quite a different failure behavior can be observed. Although the mean value of the Weibull distribution is markedly low-voltage lamps (up to 60 V), the field of use of the invention is practically unlimited. In high- and medium- voltage lamps (more than 60 V operating voltage), the invention can be used without restriction only for tubular lamps.
With lamps pinched on one end, however, care must be taken to assure that sparkovers between current-carrying parts be avoided.
The maximum cold fill pressure that is attainable without convection is generally markedly higher in high-voltage lamps than in low-voltage lamps, specifically being between 1 and 5 bar. This is due to the generally larger dimensions of these lamps. In the8case of high-voltage lamps, the intended "convection-free", mode of operation is therefore advantageous only if certain disadvantages can be overcome. In general, it has in fact been demonstrated that lamps operated convection-free exhibit a markedly lesser dependency on the mounting location (for instance, this is of interest for tubular lamps). The background for this is the improved uniformity of temperature distribution at the bulb found in convection-free lamps (a 50% improvement in variance is typical) and in general the trend to lower luminous element and bulb temperatures. Both effects lead to an improvement in the service life.
As a consequence, such lamps can also especially advantageously be used in reflectors or light fixtures. This is true for both high-voltage and low-voltage versions. A reduction in the temperature load by 10%, measured at the lamp pinch, over "high-pressure lamps" is typical.
By suitable geometrical dimensioning of the lamp it is also possible in the mode of the convection-free fill pressure range, instead of a simple linear dependency of the power losses (when plotted logarithmically), to create one region of linear dependency at low pressures and one region (plateau) at higher pressures, in which the power loss is virtually independent of the pressure a longitudinal axis of the luminous element is defined. The luminous element may be located either parallel or vertical to the end of the bulb, which is typically closed with a pinch seal. The bulb shape may be cylindrical but may also assume some other shape. Typical internal dimensions (the inside diameter in the case of the cylindrical shape, for instance) are between 3 and 15 mm, but larger values are possible as well. Examples of photoelectric data are from 10 to more than 20 lm/W (minimum value 1 m/W) for a service life of 2000 hours.
The advantages of the invention are expressed particularly markedly in low-voltage, low-power lamps. The fill volume of the bulb in low-voltage lamps is on the order of magnitude of 0.05 to 1 cm.sup.3 and in high-voltage lamps up to 15 cm.sup.3. A noble gas, optionally with the admixture of nitrogen, is used as the fill gas. A typical cold fill pressure is 0.5 to 1.7 bar in low- voltage lamps and up to 5 bar in high-voltage lamps. Halogenated hydrocarbons are examples of suitable halogen compounds.
The luminous element dimensions themselves also have an influence on operating performance. For instance, the enveloping cylinder of the luminous element should advantageously be shaped such that its length is at least equal to the diameter; in particular, it can be more than 1.5 times and preferably more than 2 times greater than the diameter.
The core factor and pitch factor of the luminous element also have an influence on the convection behavior. A point of departure for the pitch factor is a value of less than 2.0. The specific value in an individual case, however, must be arrived at empirically.
The color temperature of the lamps of the invention is in a range from about 2400 to 3400K. In general, it is true that the present invention is advantageously applicable both to lamps closed on one end and lamps closed on both ends. The closure is typically done by pinching, but can optionally also be done by fusing. In lower (typically 4000 hours), the variance in the mean service life is so sharply reduced that the 3% value (that is, the service life until the first 3% of the lamps fail) can nevertheless be equally favorable or even more favorable than in lamps according to the known prior art. This means that the time period between the failure of 3% of the set of lamps and the failure of 63.2% of the set is sharply reduced, corresponding to a substantially steeper slope of the Weibull distribution. Thus despite a poorer mean service life, an approximately equivalent or even more- effectively usable service life (hereinafter called the rated service life) can be attained (defined as the time period defined by the service life of the first 3% of a set of lamps).
A further particular advantage of the invention is that under the operating conditions according to the invention, faceting of the surface of the coiled wire can be avoided. Faceting describes the phenomenon in which the individual grains of the wire material in lamp operation begin to grow in accordance with their actually cubic space-centered lattice structure. As a result, for one thing the wire surface becomes uneven, and for the other the radiant wire surface area is enlarged. This process generally leads to a reduction in the residual light flux that is normally measured after 75% of the rated service life. Lamps according to the invention now exhibit the astonishing phenomenon that the residual light flux is markedly greater than in comparison lamps. An increase in the light flux over the initial value can even occur. The cause is suspected again to be the tungsten migration along the surface, which has a smoothing and equalizing effect on the surface profile.
The incandescent halogen lamps according to the invention normally have a high-temperature-proof bulb of quartz glass or hard glass. The luminous element contained in it is shaped (typically as a single or double coil) cylindrically or at least approximately cylindrically (a slightly curved cylinder, for instance), so that range. This plateau behavior is especially suitable for attaining the operating state at relatively low pressure according to the invention even in high-voltage lamps. This is because the plateau makes it possible to choose the "operating point" not solely preferentially just below the transition point at which the convection ensues but rather to adjust the pressure to a markedly lower value within the plateau or also at the beginning of the plateau.
In general, the risk of sparkover in high-voltage lamps can be varied by means of a fill gas mixture of noble gas with a small (up to about 10%) addition of nitrogen.
Further advantages of the invention are reduced gas consumption (which is especially significant when expensive xenon is used) and increased security against bursting.
Finally, in terms of the strain on the coil, it should be noted that high-pressure lamps (that is, lamps impinged upon by convection) not only have a markedly higher coil temperature but also a smaller radiant surface area than corresponding low-pressure lamps (that is, operated convection-free) having the same photometric data.
Low-pressure halogen lamps overall exhibit quite a different failure behavior, similar to that of conventional incandescent lamps, from high-pressure halogen lamps. "Low pressure" here generally always means the pressure that assures the freedom from convection, in comparison with the higher pressure ("high pressure,") involving convection, for a particular lamp type. While in one lamp type a cold fill pressure of 2 bar can still belong to the "low-pressure" range, in another lamp type a cold fill pressure of 0.8 bar can already be considered part of the "high-pressure range". A reliable statement about this for each lamp type can be made by measurement of the power loss as a function of the fill pressure and by determination of the transition point.